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The human body has more than 600 muscles that help us perform a wide range of movements and functions. One such muscle is the lateral pterygoid, a craniomandibular muscle that plays a crucial role in the inferior temporal region. This muscle is responsible for depressing the mandible, or jawbone, and is the only muscle of mastication that causes this depression. The lateral pterygoid is a triangular muscle with two heads or bellies, and its function depends on the degree of its contraction. When the muscle contracts bilaterally, the mandible moves forward, and during unilateral contraction, the mandible moves to the opposite side.
| Characteristics | Values |
|---|---|
| Muscle that depresses the mandible | Lateral pterygoid muscle |
| Muscle that depresses the hyoid bone | Suprahyoid muscle group (including the digastric, mylohyoid, and geniohyoid muscles) |
| Muscle that depresses the scapula | Pectoralis minor |
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What You'll Learn
The lateral pterygoid muscle depresses the mandible
The lateral pterygoid muscle is a craniomandibular muscle that plays a crucial role in the inferior temporal region. It is one of the four main muscles of mastication, which are responsible for the chewing movement of the mandible at the temporomandibular joint (TMJ). These muscles enhance the process of eating and assist in grinding food. The lateral pterygoid muscle is unique among the four muscles of mastication as it is the only one that participates in depressing the mandible.
The lateral pterygoid muscle has two heads or bellies: the superior belly and the inferior belly. The superior belly was mostly believed to become active during the elevation of the mandible to close the mouth. However, recent studies suggest that the superior belly may have little role in closing the mouth and may instead become active during mouth opening, just like the inferior belly. The inferior belly is three times larger than the superior belly, and their functions are nearly opposite.
The inferior lateral pterygoid arises from the lateral border of the lateral pterygoid plate and inserts into the pterygoid fovea on the neck of the mandible. The superior lateral pterygoid arises at the infratemporal surface of the greater sphenoid wing of the sphenoid bone and inserts on the articular capsule, the disc, and the neck of the condyle. The primary function of the lateral pterygoid muscle is to pull the head of the condyle out of the mandibular fossa along the articular eminence to protrude the mandible.
Bilateral contraction of the lateral pterygoid muscles results in protrusion and depression of the mandible. A unilateral contraction on a particular side, in conjunction with the ipsilateral medial pterygoid muscle, moves the mandible to the opposite side. The lateral pterygoid muscle assists with side-to-side mandibular motion to help with the grinding of food. It is active during mandibular movements, including protrusion (forward movement of the mandible), abduction (depression of the mandible), and mediotrusion (mandibular condyle movement towards the midline).
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The subclavius muscle depresses the lateral end of the clavicle
The subclavius muscle is a small, triangular muscle that lies underneath the clavicle, originating from the first rib. It plays a crucial role in stabilising the clavicle and preventing dislocation at the sternoclavicular joint. Additionally, it helps protect the neurovascular structures beneath it in the event of a clavicular fracture.
The main function of the subclavius muscle is the active stabilisation of the clavicle during movements of the shoulder and arm. Its contraction leads to the depression of the sternal (medial) end of the clavicle and the subsequent elevation of the first rib. This movement, although considered insignificant, is essential in preventing clavicle dislocation during certain activities.
The subclavius muscle arises from the first rib by a short, thick tendon. The fleshy fibres then proceed obliquely superolaterally to insert into a groove on the undersurface of the middle third of the clavicle. This insertion point is known as the groove for the subclavius muscle. The muscle is enclosed by the clavipectoral fascia, a fibrous connective tissue that spans the interval between the clavicle and pectoralis minor muscle.
While the subclavius muscle primarily depresses the sternal end of the clavicle, it is worth noting that it also contributes to controlling the position of the scapula in relation to the thorax through its lateral aponeurotic extension and fascial connections. This control helps regulate the output of kinetic energy during the upper extremity's movement.
In summary, the subclavius muscle, with its origin and insertion on the first rib and clavicle, respectively, plays a vital role in stabilising the clavicle and preventing dislocation. Its contraction results in the depression of the sternal end of the clavicle, ensuring the smooth and safe movement of the shoulder and arm.
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The deltoid muscle depresses the arm
The deltoid muscle is a large, triangular-shaped muscle that covers the top of the shoulder. It is responsible for lifting the arm and providing the shoulder with its range of motion. Deltoid muscles help move the arms in different directions, including to the front, side, and back. They also work with other shoulder muscles, such as the rotator cuff muscles, to enable a variety of movements.
The deltoid muscle is composed of three distinct sections: the anterior or clavicular, middle or acromial, and posterior or spinal. The anterior deltoid is responsible for flexion, internal rotation, and horizontal adduction. To stretch this muscle, the movement is reversed by extension, external rotation, and horizontal abduction of the shoulder. The posterior deltoid is involved in extension, external rotation, and horizontal abduction. To stretch the posterior deltoid, the actions are reversed to flexion, internal rotation, and horizontal adduction.
The deltoid muscle is susceptible to various injuries and conditions, including rotator cuff tears, shoulder impingement syndrome, shoulder separation, strains, and overuse injuries. Deltoid muscle pain can occur in individuals who perform repetitive overhead arm movements, such as swimmers, pitchers, or athletes.
Deltoid strengthening exercises are recommended to improve the muscle's strength and prevent injuries. These exercises include wall push exercises, dumbbell weight arm raises, and resistance band exercises.
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The thenar muscle group depresses the thumb
The thenar muscle group is a collection of three short muscles located at the base of the thumb. These muscles produce a bulge, known as the thenar eminence. The thenar eminence is responsible for the fine movements of the thumb, such as grabbing and pinching.
The three muscles that make up the thenar eminence are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The abductor pollicis brevis is the most superficial muscle of the group, originating from the tubercles of the scaphoid and trapezium bones, as well as the flexor retinaculum. It receives its innervation from the recurrent (thenar) branch of the median nerve. The flexor pollicis brevis is the most medial of the thenar muscles, with two heads (superficial and deep) separated by the tendon of flexor pollicis longus. The opponens pollicis is the largest of the thenar muscles and lies underneath the other two. It originates from the tubercle of the trapezium and the associated flexor retinaculum and inserts onto the lateral margin of the first metacarpal.
The prime function of the flexor pollicis brevis muscle is to produce flexion of the thumb at the metacarpophalangeal and carpometacarpal joints, as well as contributing to the medial rotation of the thumb. The abductor pollicis brevis and the flexor pollicis brevis allow for the movement of the thumb away from or towards the other fingers. The opponens pollicis enables the thumb to be opposable, allowing for complex movements that combine flexion, adduction, and medial rotation. This movement is crucial for fine motor skills and precise hand movements such as writing or pinching.
While the adductor pollicis is not part of the thenar muscle group, it is sometimes described together with the thenar muscles when discussing thumb movements. It is a short and broad, fan-shaped muscle of the palm, arising from two muscular heads: the oblique and transverse. The median nerve innervates all the thenar muscles, except for the deep head of flexor pollicis brevis, which, along with the adductor pollicis muscle, receives innervation via the ulnar nerve.
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The coracobrachialis muscle depresses the arm at the shoulder
The coracobrachialis muscle is a small muscle located in the upper medial part of the arm, originating from the coracoid process of the scapula and inserting onto the middle of the medial aspect of the body of the humerus. It is functionally weak and insignificant, but it plays a supporting role in adduction and flexion of the arm at the shoulder joint. Adduction refers to the movement of bringing the arm towards the trunk, while flexion involves bending the arm.
The coracobrachialis muscle is classified into distinct superficial and deep layers, with some individuals exhibiting a fully divided or partially separated muscle. It is innervated by the musculocutaneous nerve, which can become trapped in cases of overuse or injury, leading to pain and sensory disturbances in the arm and hand. This muscle also resists deviation of the arm from the frontal plane during abduction.
The main function of the coracobrachialis is to produce flexion and adduction, along with other flexors of the arm such as the biceps brachii and brachialis muscles. It acts as a strong antagonist to the deltoid muscle when the arm is abducted and extended. The muscle is involved in internal rotation, bringing the arms inward towards the body, and contributes to shoulder stability by holding the upper arm bone within the shoulder socket during rotation.
The coracobrachialis can be strained or tightened by certain activities, such as push-ups, swinging on gymnastic rings, or carrying heavy loads. It is important to be mindful of overuse to prevent muscle pain, stiffness, or injury. Overall, while the coracobrachialis muscle is not a prominent arm muscle, it plays a supporting role in various arm movements and shoulder stability.
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Frequently asked questions
The lateral pterygoid muscle is the only muscle that participates in depressing the T-shaped mandible.
The lateral pterygoid muscle is a craniomandibular muscle that has two heads or bellies with distinct origins. It is active during mastication and mandibular movements, including protrusion, abduction, and mediotrusion.
The lateral pterygoid muscle depresses the mandible when assisted by the anterior belly of the digastric muscle and the mylohyoid muscle. The unilateral contraction of the lateral pterygoid muscle with the ipsilateral medial pterygoid muscle results in lateral mandibular movement to the contralateral side.
The lateral pterygoid muscle also assists with protrusion and side-to-side movement of the mandible. It is active during speaking, singing, and clenching.
